1. Field of the Invention
The present invention relates to an apparatus for estimating the state of charge of a rechargeable battery charged by a vehicle-mounted generator which outputs a variable voltage.
2. Description of Related Art
As such an estimating apparatus, Japanese Patent Application Laid-open No. 2004-168126 discloses an apparatus configured to perform a process to periodically change the output voltage of an alternator driven by an internal combustion engine between a first voltage and a second voltage lower than the first voltage, and determine that a battery has reached a predetermined state of charge if the value of a charging current of the battery becomes a predetermined value after the output voltage of the alternator was set last time to the first voltage. Japanese Patent Application Laid-open No. 2003-307557 also discloses such an estimating apparatus.
Meanwhile, there has been proposed a control method in which the output voltage of an alternator is set larger when an increase amount of fuel consumption of an internal combustion engine needed for the alternator to generate power is smaller, with the object of reducing fuel consumption of the internal combustion engine. In performing such a control method, it is desired that, when the output voltage of an alternator is set to a small value to facilitate discharge of a battery, the battery is appropriately discharged within limits to maintain the reliability of the battery. To this end, it is required to estimate the state of charge of the battery with high degree of accuracy.
However, there is a problem in using the conventional estimating apparatus as described above. That is, in case the state of charge of a battery is estimated by use of the estimating apparatus, and thereafter, the estimated state of the battery is updated in accordance with an integrated value of the charging/discharging current of the battery, since the accuracy of the estimated state of charge of the battery is low, the fuel consumption cannot be sufficiently reduced. It may occur that the estimating apparatus estimates the state of charge of the battery at shorter intervals. However, in this case, since the output voltage control of the alternator for the estimation purpose has to be performed more frequently, the output voltage control of the alternator for the purpose of reducing fuel consumption may be prevented from being performed.